Daisychain LG Ultrafine 4K and 5K

[fakenick]

Hi,

I have an LG Ultrafine 4K (24") and an LG Ultrafine 5K monitor. If I attach my MacBook Pro to the LG 4K, and daisy chain the LG 4K to the LG 5K, it appears my LG 5K will only run at 4K resolution.

I'm using a third party cable and unsure if its fully thunderbolt 3 compliant, or whether this is a bandwidth issue.

Can anyone advise whether it's possible to daisy chain these two monitors and have them run at their maximum resolutions?

Thanks

 

[joevt]

Thunderbolt only has two DisplayPort connections - one of them is used for the LG UltraFine 4K leaving only one DisplayPort connection for the LG UltraFine 5K. The LG UltraFine 5K requires two DisplayPort connections over Thunderbolt to achieve 5K60 (one connection for each 2560x2880 tile of the display).

You can try the LG UltraFine 5K using a custom timing of 5K 39Hz.

Or you can replace the LG UltraFine 5K with an Apple Studio Display. The Apple Studio Display can achieve 5K60 with a single DisplayPort connection if your GPU supports DSC.

 

[fakenick]

Thanks very much for your knowledgeable reply, hadn’t seen this written anywhere else. I suspected this was too much for thunderbolt 3 to handle, interesting that the studio display is more efficient though!

I think I’ll stick to plugging in two cables rather than splashing out on a studio display 😀

 

[chrfr]

Thanks very much for your knowledgeable reply, hadn’t seen this written anywhere else. I suspected this was too much for thunderbolt 3 to handle, interesting that the studio display is more efficient though!

I think I’ll stick to plugging in two cables rather than splashing out on a studio display 😀

It's less that Thunderbolt can't handle it than it is that the LG 5k is dated technology.

 

[brosenz]

Hi,

I have an LG Ultrafine 4K (24") and an LG Ultrafine 5K monitor. If I attach my MacBook Pro to the LG 4K, and daisy chain the LG 4K to the LG 5K, it appears my LG 5K will only run at 4K resolution.

I'm using a third party cable and unsure if its fully thunderbolt 3 compliant, or whether this is a bandwidth issue.

Can anyone advise whether it's possible to daisy chain these two monitors and have them run at their maximum resolutions?

Thanks

I have the same configuration with a MBP 16M1, 4K directly connected to the MBP, and 5K daisy chain to the 4K monitor, the Ultrafine 4K runs at 1920x1080 and the 5K runs at 2048x1152, they both look fine, I am not sure if that's 4K or 5K resolution.

 

[joevt]

I have the same configuration with a MBP 16M1, 4K directly connected to the MBP, and 5K daisy chain to the 4K monitor, the Ultrafine 4K runs at 1920x1080 and the 5K runs at 2048x1152, they both look fine, I am not sure if that's 4K or 5K resolution.

LG UltraFine 5K can't do 5K60 from a Thunderbolt port that has another display connected to it.

macOS only shows the frame buffer resolution of the displays. It does not show the output resolution that is being sent to the display. You need to use a utility like SwitchResX to view the timing info of the currently selected display resolution. Double-click the currently selected display resolution in the Current Resolutions tab of SwitchResX. It will show the pixel clock and active pixels. For 5K it should show pixel clock > 900 MHz and active pixels 5120x2880.

If you only want to use 2048x1152 then I suppose it doesn't matter since there's little difference between having the display scale 4096 x 2304 to 5120 x 2880 (what you are seeing now) vs having the GPU scale 4096 x 2304 to 5120 x 2880 and in fact you might be getting slightly better performance by not having the GPU do the scaling.

However, if you want to use "Looks like 2560 x 1440", then you'll definitely want to connect the LG UltraFine 5K to a separate Thunderbolt port because otherwise you'll have 5120 x 2880 being scaled down to 4096 x 2304 by the GPU and scaled up to 5120 x 2880 by the display.